A wheel brake actuator is known which is actuated by electric motor and includes a pretensioned spring element and a drum brake system of the duo-servo type for a motor vehicle, and requires an operating method which detects or senses significant changes in the power drain of the electric motor actuator during the application and release of the brakes in order to determine specific values of the arresting force using the spring element (DE 10 2007 035 541 A1).
The parking operation of a vehicle with a hot wheel brake, in particular on a slope, can result in changes in the brake application force owing to cooling. In this context, disc brakes generally experience a loss of brake application force, while drum brakes are basically subject to an increase in brake application force. In drum brakes, there is an additional aggravating factor that for structural reasons said brakes may have self-boosting effects which are different depending on the direction of rotation, which become manifest during the cooling of the wheel brake with respect to the change in the brake application force in an opposite direction.
In addition, in the case of duo-servo drum brake systems, in particular stick-slip effects are known in which the brake shoes temporarily adhere to or slip on the brake drum. These facts can cause undesired irregularities in the detection of current, which makes filtering necessary.
In order to implement a quasi-elastic force effect of the brake shoes on the brake drum in conjunction with a comprehensive monitoring function, a specific central actuating unit is known, with an activation cable and two couplings to wheel brakes, in which the spring-elastic lengthening of the actuation cable is utilized, as it were, as a reversible energy store by the central actuating unit (EP 966 376 B1). However, in such systems, two sensors are prescribed for monitoring the activation cable, and correspondingly extensive signal processing operations in a control unit are prescribed.
In order to prevent an undesired release of the brakes, it has to a certain extent also already been proposed to monitor automatically, that is to say independently of the driver, a vehicle state or the state of the wheel brake at least within defined time periods after the parking operation—what is referred to as the overrun time—by means of an electronic control unit, and where necessary to retension the parking brake actuator system automatically. It goes without saying that this automatic operation additionally places demands on the electrical power supply, that is to say on the vehicle on-board power system, which can actually be tolerated only in conjunction with an unlimited electrical energy supply.
The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.